Do neurons from rat neostriatum express both a TTX-sensitive and a TTX-insensitive slow Na+ current?

1. The properties of a tetrodotoxin (TTX)-sensitive, persistent Na+ current and a purported TTX-insensitive slow Na+ current were studied in acutely isolated neurons from rat neostriatum with the use of the whole cell configuration of the patch-clamp technique. 2. A TTX-sensitive, persistent Na+ current (INaP) was activated positive to -60 mV and reached a peak amplitude of -40 to -120 pA at about -40 mV. As indicated by slow depolarizing voltage ramps, activation of INaP did not require preceding activation of the fast, rapidly inactivating Na+ current. 3. The current-voltage (I-V) relationship of INaP displayed an unexpected inflection after passing through its peak value near -40 mV. Between -40 and -10 mV, INaP declined more rapidly with depolarization than it did at more depolarized potentials. The corresponding conductance (GNaP) peaked at -40 mV and declined to a smaller limiting value at potentials positive to about -10 mV. 4. This behavior is not consistent with the notion that INaP arises solely from a bell-shaped window conductance that results from the overlapping steady-state activation and inactivation curves of the fast Na+ current in a narrow voltage range, nor with the notion that INaP is generated by a single uniform conductance independent of the fast Na+ current. 5. In addition to INaP, a second slow inward current (IS) was evoked when small monovalent cations were omitted from the internal solution. INaP and IS were present both in cells resembling medium spiny neurons and in cells resembling aspiny interneurons. 6. IS was insensitive to TTX (1.2 microM) and the Ca2+ channel blocker, cadmium.(ABSTRACT TRUNCATED AT 250 WORDS)